Utilization of total knee replacement (TKR) is on the rise and projections of a 6-fold increase over the next 2 decades have alarmed policy makers, with several initiatives aimed at restraining costs.1 Yet, these projections do not dismiss TKR underuse, which was strongly endorsed by an NIH consensus panel in 2004 and is often discussed in the literature and acknowledged by policy makers.2 Starting in 2014, insurance expansion under the Affordable Care Act will include more than 50 million low-income Americans,1 among whom are subpopulations who have been reported to underuse TKR.3 Simply providing insurance will not fully address the utilization gap.4, 5 Factors associated with underuse and overuse need to be identified so that policies to address both phenomena can be developed in the context of cost control pressures. Little has been done to investigate the extent of underuse and overuse of TKR. The available literature on disparities in TKR utilization (e.g. between whites and African Americans) partially addresses this issue. However, because this literature is focused on actual surgeries, it fails to answer questions like: Is the gap in utilizaion between Whites and African-Americans due entirely to the African-Americans underutilizing the surgery or is part of it Whites overusing the surgery? By utilizing the Osteoarthritis Initiative (OAI) and Multicenter Osteoarthritis Study (MOST) longitudinal datasets of almost 8,000 persons with or at risk for knee osteoarthritis, this proposal takes a prospective approach to studying underuse and overuse of knee replacement in the US. We will apply appropriateness classification systems, which are evidence-based metrics used to assess both overuse and underuse of surgical procedures, to study utilization of TKR.6 We propose to apply the system to OAI and MOST subjects with or at risk of knee osteoarthritis to estimate rates of timely use, overuse and underuse. We then study the predictors of overuse and underuse of TKR. This study will be the first to prospectively assess the extent of underuse and overuse of TKR in the US. Results will help public and private insurers to consider underuse and overuse in developing reimbursement policies and to design along with providers interventions aimed at under and over users. In addition, findings from this project will provide critically important information for developing consensus-based evidence-based appropriateness criteria in the US.